1. Field of the Invention
The present invention relates to a gas furnace, and more particularly, to a method for controlling an air fuel ratio in a gas furnace.
2. Background of the Related Art
FIG. 1 illustrates a construction of a general gas furnace. Referring to FIG. 1, the general gas furnace includes a burner 11 for mixing air with fuel gas and burning the mixture, an igniter for igniting the mixture of the air and the fuel gas, a flame sensor for sensing flame, a gas valve 12 for supplying and excluding the fuel gas, an inducer fan 13 for supplying the combustion air to the burner 11, a first heat exchanger 14 and a second heat exchanger 15 for heating outside air through the burning operation of the burner 11, a blower 16 for circulating the air to transfer the heat transferred through the first and second heat exchangers 14 and 15 to the inside of a room, a controller (not shown) for controlling operation of each component of the gas furnace according to a user""s operation command or a preset operation condition, and a memory (not shown).
A heating cycle of the gas furnace constructed as above will be explained herein below with reference to FIG. 2.
Firstly, once the controller is turned on, it drives the inducer fan 13 for a predetermined period of time and performs a pre purge stroke to discharge a residual gas or the likes remaining during a pre-burning operation.
The controller supplies a power source to the igniter to heat the igniter for a predetermined period of time. In a state that an ignition condition is satisfied, the controller opens the gas valve 12 and supplies the fuel gas to the burner 11.
After the controller opens the gas valve 12 for the predetermined period of time, it cuts off the supply of the power source to the igniter.
At the same time, the controller senses flame by means of the flame sensor to know whether the ignition is exactly carried out for the period of time after the gas valve 12 is opened. If there is sensed the flame, the controller turns on the blower 16 after a predetermined period of time, so as to proceed with a normal combustion.
The controller measures an indoor temperature through a temperature sensor. If the temperature reaches a preset temperature, the controller closes the gas valve 12 to extinguish fire, and drives the inducer fan 13 and the blower 16 for a predetermined period of time to perform a post purge stroke for the purpose of cooling the inside of the gas furnace and discharging a residual gas inside the gas furnace.
Next, the controller turns off the inducer fan 13, and drives only the blower 16 for a predetermined period of time so as to complete the fire extinguishing.
The controller performs the heating operation by reiterating the above burning and extinguishing process according to the preset heating temperature condition and the user""s operation command.
A method for controlling an air fuel ratio in the gas furnace of the conventional art will be explained herein below.
The conventional gas furnace is provided with one or more temperature control modes, such as strong/intermediate/weak modes, etc., so as for the user to select a desired temperature.
The strong/intermediate/weak modes are set to have corresponding calorific values: 6000 Kcal/hour for the strong mode, 5000 kcal/hour for the intermediate mode, and 3000 Kcal/hour for the weak mode. An air fuel ratio suitable for each calorific value has been set through experiments. Thus, if the user selects the strong mode, the gas valve 12 and the inducer fan 13 are controlled to supply the air and the fuel at the preset air fuel ratio which is suitable for generating heat of 6000 Kcal/hour. Then, if the user changes the temperature to the weak mode, the gas valve 12 and the inducer fan 13 are controlled to supply the air and the fuel at the preset air fuel ratio which is suitable for generating heat of 3000 Kcal/hour.
At this time, gas fuel quantity is controlled in a manner that magnetic field strength of a solenoid in the inside is varied according to a voltage supplied to the gas valve 12 and opening of the gas valve 12 is accordingly varied. Air quantity is controlled by a revolution of a fan motor which drives the inducer fan 13. Therefore, the controller has to detect the revolution to control the revolution of the fan motor. The process for detecting the revolution will be explained herein below.
The controller applies an operation voltage to the fan motor to drive the fan motor, which is accordingly operated after receiving the operation voltage. As the fan motor is operated, pulse signals are generated in a pulse signal generating part connected to the fan motor in proportion to the revolution.
Therefore, as shown in FIG. 3a, the number of pulses output in the pulse signal generating part of the fan motor is counted at constant time intervals, namely every second, to detect the revolution of the fan motor, whereby the revolution of the fan motor is renewed every second.
Meantime, in the event that the gas combustion condition is drastically changed, for example, the mode is changed from the weak mode to the strong mode, as shown in FIG. 3b, pulse recurrence intervals are varied several times for a second. As a result, the number of pulses is varied, and the revolution per minute (RPM) of the fan motor is also varied. In this case, the fan motor RPM cannot be exactly detected.
In the gas furnace of the conventional art, if the temperature control stage is varied, the air fuel ratio is varied instantaneously in accordance with the varied temperature control stage. That is to say, since the fan motor RPM has to be rapidly varied and the opening of the gas valve has to be rapidly varied to supply the air and the gas at the varied air fuel ratio, there inevitably exists a transient state, namely a state that the air fuel ratio is improper, wherein the combustion is unstable, excessive exhaust gas is generated, and an abnormal termination of the combustion is occurred because of environmental conditions and factors, thereby deteriorating performance in combustion.
Accordingly, the present invention is directed to a method for controlling an air fuel ratio in a gas furnace that substantially obviates one or more problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for controlling an air fuel ratio in a gas furnace, which can maintain optimum performance in combustion irrespective of variation in temperature control stages.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a method for controlling an air fuel ratio in a gas furnace which drives a fan motor with a specified voltage, measures a cycle time of pulse signals generated during revolution of the fan motor, detects the number of revolutions of the fan motor according to the measured cycle time, and controls opening of a gas valve based on an average voltage of a PWM(Pulse Width Modulation) signal applied from a controller, the method including the steps of detecting data of variation in calorific value according to variation in a PWM count value, and data of variation in air quantity according to variation in the RPM(Revolution Per Minute) of the fan motor; detecting data of variation in the PWM count value according to the variation in the fan motor RPM by using the above calorific value variation data and the air quantity variation data variation data, and deriving a relational expression between the fan motor RPM and the PWM count value by using the PWM signal count value variation data; and applying the detected RPM to the relational expression until a calorific value reaches a level preset in a temperature control mode selected by a user during a burning operation of the gas furnace, and accordingly controlling a gas valve with the PWM count value.
It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.